Max Planck Institute for Ornithology, Seewiesen, Germany.
PLoS One. 2011;6(8):e23203. doi: 10.1371/journal.pone.0023203. Epub 2011 Aug 24.
Mammals and birds engage in two distinct states of sleep, slow wave sleep (SWS) and rapid eye movement (REM) sleep. SWS is characterized by slow, high amplitude brain waves, while REM sleep is characterized by fast, low amplitude waves, known as activation, occurring with rapid eye movements and reduced muscle tone. However, monotremes (platypuses and echidnas), the most basal (or 'ancient') group of living mammals, show only a single sleep state that combines elements of SWS and REM sleep, suggesting that these states became temporally segregated in the common ancestor to marsupial and eutherian mammals. Whether sleep in basal birds resembles that of monotremes or other mammals and birds is unknown. Here, we provide the first description of brain activity during sleep in ostriches (Struthio camelus), a member of the most basal group of living birds. We found that the brain activity of sleeping ostriches is unique. Episodes of REM sleep were delineated by rapid eye movements, reduced muscle tone, and head movements, similar to those observed in other birds and mammals engaged in REM sleep; however, during REM sleep in ostriches, forebrain activity would flip between REM sleep-like activation and SWS-like slow waves, the latter reminiscent of sleep in the platypus. Moreover, the amount of REM sleep in ostriches is greater than in any other bird, just as in platypuses, which have more REM sleep than other mammals. These findings reveal a recurring sequence of steps in the evolution of sleep in which SWS and REM sleep arose from a single heterogeneous state that became temporally segregated into two distinct states. This common trajectory suggests that forebrain activation during REM sleep is an evolutionarily new feature, presumably involved in performing new sleep functions not found in more basal animals.
哺乳动物和鸟类都有两种截然不同的睡眠状态，即慢波睡眠（SWS）和快速眼动（REM）睡眠。SWS 的特征是大脑电波缓慢而高振幅，而 REM 睡眠的特征是快速而低振幅的波，称为激活，伴随着快速的眼球运动和肌肉张力降低。然而，单孔目动物（鸭嘴兽和针鼹），是现存最基础（或“古老”）的哺乳动物群体，只表现出一种结合了 SWS 和 REM 睡眠元素的单一睡眠状态，这表明这些状态在有袋类和真兽类哺乳动物的共同祖先中就已经出现了时间上的分离。基础鸟类的睡眠是否类似于单孔目动物或其他哺乳动物和鸟类尚不清楚。在这里，我们首次描述了鸵鸟（Struthio camelus）睡眠期间的大脑活动，鸵鸟是现存最基础鸟类群体的一员。我们发现，鸵鸟的睡眠脑活动是独特的。REM 睡眠的发作是通过快速眼球运动、肌肉张力降低和头部运动来界定的，类似于其他鸟类和哺乳动物的 REM 睡眠；然而，在鸵鸟的 REM 睡眠中，前脑活动会在 REM 睡眠样激活和 SWS 样慢波之间切换，后者让人联想到鸭嘴兽的睡眠。此外，鸵鸟的 REM 睡眠量比任何其他鸟类都多，就像鸭嘴兽一样，它们的 REM 睡眠比其他哺乳动物多。这些发现揭示了睡眠进化中反复出现的一系列步骤，在这个过程中，SWS 和 REM 睡眠从一种单一的异质状态中产生，这种状态在时间上被分离成两种截然不同的状态。这种共同的轨迹表明，REM 睡眠期间的大脑前区激活是一种新的进化特征，可能涉及执行在更基础的动物中未发现的新的睡眠功能。
